What is HCDP (Hydrocarbon Dew Point)
Principles of HCDP | Why Control HCDP | Specifications for HCDP | Cricondentherm Temperature |
Hydrocarbon Gas Dew Point Curve


Hydrocarbon Gas Dew Point Curve

Dew Point Temperature
Pressure
F
PSIA
9.0°
200
12.9°
250
15.6°
300
17.5°
350
18.8°
400
19.5°
450
19.7°
500
Cricondentherm
19.6°
550
19.1°
600
18.2°
650
17.0°
700
15.4°
750
13.5°
800
11.1°
850
Operating at 9°F and 875#
8.5°
900
5.2°
950
1.4°
1000
-3.3°
1050
-9.4°
1100
-18.7°
1150

The temperature shown in the HDP curve represents the gas dew point at the corresponding pressures.

A cricondentherm specification at first seems like the best way a pipeline can protect its assets. The transporting pipeline operator knows if it sets a cricondentherm temperature restriction below the lowest temperature seen in its system, it can raise and lower the gas pressure in the pipeline transportation system, and not have to worry about liquid condensation.

The problem a pipeline operator has in using a cricondentherm specification is in the calculation of the cricondentherm temperature. The cricondentherm temperature is calculated by obtaining an extended gas analysis and then inputting the analysis data into a software package, using equations of state to predict the dew point temperatures at the range of pressures.

However, many gas-transporting companies tend to collect gas composition data using on-line chromatographs or composite samples with a grouped C6+ component. The C6+ component does not provide any information on the heavier hydrocarbon (C7+) components that determine the gas hydrocarbon dew point. To calculate a cricondentherm the pipeline operator must make some assumptions. It is these assumptions that are causing problems. The pipeline operator must decide how to distribute the C6+ component for his calculation. The most commonly used distribution assumptions are the Daniels/El Paso distribution (i.e. 48% C6; 35% C7; 17% C8+) and the GPA distribution (i.e. 60% C6, 30% C7, 10% C8+). If the Daniels distribution shown in the previous sentence is used on the gas represented in the dew point curve above, the cricondentherm dew point calculates to be 38.1°F, which is 18.4°F higher than its actual cricondentherm temperature. The producer would need to operate his cold separator on his conditioning unit at -10°F (negative 10°F) to meet the system requirements due to the assumptions made in calculating the gas cricondentherm. Another popular mistake is to perform an analysis that groups the C6, C7 and C8+ components, rather than using the detailed component-by-component breakdown. Grouping also skews the cricondentherm. If you group the above components, the cricondentherm calculates at 32.6°F or 12.9°F high. It is DPC's experience that grouping will add a minimum of 3°F to 5°F to the calculated cricondentherm temperature.

To be useful in a commercial environment, pipeline hydrocarbon dew point specifications must be easily measured with existing equipment. The majority of the transporting pipeline systems measure using a C6+ component system. These systems can be used to track cricondentherm based specifications as long as the heavier components are not distributed arbitrarily.

DPC does not recommend detailed analysis be taken beyond C8+ on dew point conditioned gas streams as it is not useful and results in unnecessary expense.